Mixed monolayers of dipalmitoylglycerophosphocholine,distearoylglycerophosphocholine and 1-palmitoylglycerol

Mixed monolayer systems of dipalmitoylglycerophosphocholine (DPPC)-distearoylglycerophosphocholine (DSPC), palmitoylglycerol-DPPC, and palmitoylglycerol-DSPC have been investigated by a previously described thermodynamic treatment. The surface pressures of these systems were measured at various compositions and temperatures. The two-dimensional phase diagrams and the apparent molar entropy and energy changes were evaluated. It was found that the phase diagrams of the DPPC-DSPC, palmitoylglycerol-DSPC and palmitoylglycerol-DPPC systems are all of different types. According to our phase diagram classification, DPPC-DSPC, palmitoylglycerol-DSPC, and palmitoylglycerol-DPPC systems exhibited a modified cigar type, a eutectic type, and a negative azeotropic type, respectively.     

Effects of molecular structure on two-dimensional phase diagram and thermodynamic quantities of mixed monolayers

Many kinds of two-component mixed monolayer systems were investigated to clarify the effect of molecular structures on the monolayer state. Two-dimensional phase diagrams and thermodynamic quantities were evaluated by correct thermodynamic analysis. The diagrams were classified into the following six types: (1) cigar type; (2) modified cigar type; (3) positive azeotropic type; (4) negative azeotropic type; (5) eutectic type; (6) complicated type. Phase diagram analysis and thermodynamic quantities, such as entropy, enthalpy and energy changes, were in good agreement with each other.     

Mixing behavior of identical molecular weight phosphatidylcholines with various chain-length differences in two-component lamellae.

It has recently been suggested that mixed-chain phosphatidylcholines with normalized chain length differences (deltaC/CL) in the range of 0.10-0.40 undergo spontaneous self-assembly in excess water at T less than Tm into the partially interdigitated bilayer and those with delta C/CL values in the range of 0.44-0.57 form, in excess water, mixed interdigitated bilayers at T less than Tm. The mixing behavior of binary mixtures of C(22):C(12)PC/C(17):C(17)PC, C(22):C(12)/C(15):C(19)PC, and C(15):C(19)PC/C(13):C(21)PC reported in this work is used to support this view. The values of delta C/CL for C(17):C(17)PC, C(15):C(19)PC, C(13):C(21)PC, and C(22):C(12)PC are 0.10, 0.15, 0.35, and 0.55, respectively. The binary mixture of C(15):C(19)PC/C(13):C(21)PC exhibits a lens-shaped phase diagram, indicating that these two identical molecular weight (MW) lipids with delta C/CL values less than 0.4 are completely miscible over the entire compositional range in both gel and liquid-crystalline phases. In contrast, the phase diagrams of C(22):C(12)PC/C(17):C(17)PC and C(22):C(12)PC/C(15):C(19)PC are eutectic, indicating immiscibility of the component lipids over a wide compositional range in the gel phase. This immiscibility of identical MW lipids in the bilayer plane can be attributed to the different packing properties of the component lipids in the bilayer at T less than Tm.     

Phase behaviour of stearic acid-stearonitrile mixtures. A thermodynamic study in bulk and at the air-water interface

The solid-liquid phase behaviour of stearic acid (SA) and stearonitrile (SN) in binary mixtures was investigated by differential scanning calorimetry (DSC), and the formation of SA-SN mixed monolayers at the air-water interface was followed by surface pressure-area (pi-A) measurements and by Brewster angle microscope (BAM) observation. The solid-liquid phase diagram is a eutectic type phase diagram, with the eutectic composition 0.90相似文献   

Thermodynamic model of cooperativity in a dimeric protein: unique and independent parameters formulation.

A model of the cooperative interaction of ligand binding to a dimeric protein is presented based upon the unique and independent parameters (UIP) thermodynamic formulation (Gutheil and McKenna, Biophys. Chem. 45 (1992) 171-179). The analysis is developed from an initial model which includes coupled conformational and ligand binding equilibria. This completely general model is then restricted to focus on conformationally mediated cooperative interactions between the ligands and the expressions for the apparent ligand binding constant and the apparent ligand-ligand interaction constant are derived. The conditions under which there is no cooperative interaction between the ligands are found as roots to a polynomial equation. Consideration of the distribution of species among the various conformational states in this general model leads to a set of inequalities which can be represented as a two dimensional plot of boundaries. By superimposing a contour plot of the value of the apparent ligand-ligand interaction constant over the plot of boundaries a complete graphical representation of this system is achieved similar to a phase diagram. It is found that the parameter space homologous to Koshland-Nemethy-Filmer type of model is most consistent with both positive and negative cooperativity in this model. The maximal amount of positive and negative cooperativity are found to be simple functions of Kc, the equilibrium constant associated with the change of a subunit and ligand from the unligated to ligated conformation. It is shown that under certain limiting conditions the apparent allosteric interaction between ligands is equal to the conformational interaction between subunits. The methods presented are generally applicable to the theoretical analysis of thermodynamic interactions in complex systems.     

Mixed monolayers of straight-chain/branched-chain phospholipids. I. Mixed monolayers of distearoyl phosphatidylcholine and diisoeicosanoyl phosphatidylcholine

The temperature dependence of the force/area isotherms of monolayer of distearoyl phosphatidylcholine (DSPC), diisoeicosanoyl phosphatidylcholine (DIEPC) and a complete mixed compositional range of these two lecithins are reported. The isotherms for DSPC closely resemble those previously reported for dipalmitoyl phosphatidylcholine but are shifted to higher temperatures by 16 degrees C. The isotherms of DIEPC, an iso-branched lecithin, show differences from these obtained for similar straight-chain lecithins in that the full condensed isotherms are more expanded, the fully expanded isotherms are more condensed and therefore the liquid expanded (LE)/liquid condensed (LC) intermediate region is significantly reduced. This means that the condensed state is more disordered and the expanded state is less disordered than the corresponding states in straight-chain lecithins. Data for the mixed films are interpreted in terms of surface pressure/mole fraction phase diagrams and both energies and entropies of compression associated with the LE/LC transition. The phase diagrams at 34.1 degrees C, 35.8 degrees C and 38.5 degrees C are all of the negative azeotropic type with the surface pressure minimum point shifting with temperature. The thermodynamic analysis indicates that from 34.1 degrees C to 38.5 degrees C the driving force for mixing changes from the entropy to the energy of the transition. It would seem that at the lower temperature the packing of the distearoyl lecithin is perturbed by the diisoeicosanoyl lecithin, while at higher temperatures the very high entropy of pure or nearly pure diisoeicosanoyl lecithin results in other mixtures having less entropy than would be expected on an ideal mixing basis.     

Mixed monolayers of fatty acids with distearoylglycerophosphocholine

Mixed monolayer states of long normal-chain fatty acids with distearoylglycerophosphocholine (DSPC) have been studied by a previously described thermodynamic treatment. The surface pressures of mixed monolayers of tetradecanoic, pentadecanoic and octadecanoic acids with DSPC were measured at various compositions and temperatures. The two-dimensional phase diagrams of both the tetradecanoic acid-DSPC and pentadecanoic acid-DSPC systems were determined. They both exhibited eutectic type phase diagrams. In the octadecanoic acid-DSPC system the two components were immiscible both in the liquid-expanded and liquid-condensed states. The apparent molar entropy and energy changes for the tetradecanoic acid-DSPC system were calculated. The values for tetradecanoic acid were negative, as is the usual case. However, the values of DSPC were positive, and their absolute values wer about $\text{1}\text{4}$ of that of tetradecanoic acid.     

Phase behaviour of stearic acid-stearonitrile mixtures: A thermodynamic study in bulk and at the air-water interface

The solid-liquid phase behaviour of stearic acid (SA) and stearonitrile (SN) in binary mixtures was investigated by differential scanning calorimetry (DSC), and the formation of SA-SN mixed monolayers at the air-water interface was followed by surface pressure-area (π-A) measurements and by Brewster angle microscope (BAM) observation. The solid-liquid phase diagram is a eutectic type phase diagram, with the eutectic composition 0.90 < X_SN < 0.95 and T^eut = 40.9 °C. The DSC results also suggest that the two components are immiscible in the solid phase but form a liquid mixture with positive deviations to the ideal behaviour. At the air-water interface, the two components form liquid condensed monolayers in the entire range of compositions, at low surface pressures, while solid mixed monolayers only form at high surface pressures for X_SN < 0.8. Thermodynamic analysis indicates that SA and SN are miscible in the liquid condensed phase, with negative deviations from the ideal behaviour. The variation of the collapse surface pressure of mixed monolayers also indicates miscibility at the air-water interface.     

Temperature- and pressure-dependent phase behavior of monoacylglycerides monoolein and monoelaidin.

We used x-ray and neutron diffraction to study the temperature- and pressure-dependent structure and phase behavior of the monoacylglycerides 1-monoelaidin (ME) and 1-monoolein (MO) in excess water. The monoacylglycerides were chosen for investigation of their phase behavior because they exhibit mesomorphic phases with one-, two-, and three-dimensional periodicity, such as lamellar, an inverted hexagonal and bicontinuous cubic phases, in a rather easily accessible temperature and pressure range. We studied the structure, stability, and transformations of the different phases over a wide temperature and pressure range, explored the epitaxial relations that exist between different phases, and established a relationship between the chemical structure of the lipid molecules and their phase behavior. For both systems, a temperature-pressure phase diagram has been determined in the temperature range from 0 to 100 degrees C at pressures from ambient up to 1400 bar, and drastic differences in phase behavior are found for the two systems. In MO-water dispersions, the cubic phase Pn3m extends over a large phase field in the T,p-plane. At temperatures above 95 degrees C, the inverted hexagonal phase is found. In the lower temperature region, a crystalline lamellar phase is induced at higher pressures. The phases found in ME-water include the lamellar crystalline Lc phase, the L beta gel phase, the L alpha liquid-crystalline phase, and two cubic phases belonging to the crystallographic space groups Im3m and Pn3m. In addition, the existence of metastable phases has been exploited. Between coexisting metastable cubic structures, a metric relationship has been found that is predicted theoretically on the basis of the curvature elastic energy approximation only.     

Molecular packing in steroid-lecithin monolayers,part II: Mixed films of cholesterol with dipalmitoylphosphatidylcholine and tetradecanoic acid

Mixed film studies of the systems cholesterol/tetradecanoic acid and cholesterol/dipalmitoylphosphatidylcholine have been carried out over the entire compositional range at 21°C. When compared on an acyl chain basis the condensing effects were found to be essentially independent of which host-lipid was utilized. The phase change of the host lipid was shifted to higher pressures, then broadened and eliminated. Maximal condensation occurred at just above 42 mol% for the cholesterol/DPPC system. In both systems the two components were initially found to be miscible at all proportions.The results are interpreted in terms of the molecular packing of cholesterol with acyl boundary layers, one significantly, one weakly affected. Maximum condensation is a result of packing that provides maximum cholesterol/acyl chain contact. Consideration is given to both long term stability of such mixed monolayers and the behaviour of the corresponding bilayer states.     

Stochastic relations between species richness and the variability of species composition

Ecological communities change over time and space, and ecologists have long suggested that biodiversity might influence the rate of change. Here we cast new light on this question by demonstrating statistical covariation between species richness and the variability of species abundances and identities within a community (compositional variability). We provide a new analytical framework for several previously published measures of ecosystem functioning and compositional variability. We derive three related variances, each of which measures compositional variability due solely to stochastic sampling processes. Our analyses show that whether relations between species richness and compositional variability are positive, negative or zero, depends on two factors. Not only does the particular variance used affect the relation, but, more importantly, the underlying determinants of species richness can strongly affect the stochastic relations between species richness and compositional variability. This analysis makes clear for the first time how species richness should correlate with important measures of community variability, even in the absence of systematic processes.     

Mixed monolayer studies of 12-hydroxyoctadecanoic acid and its esters

Mixed monolayers of two bipolar/bipolar systems have been studied and the results are compared with those obtained from similar monopolar/monopolar systems. DL-12-Hydroxyoctadecanoic acid, methyl DL-12-hydroxyoctadecanoate, and ethyl DL-12-hydroxyoctadecanoate were used as the bipolar substances in this paper. The surface pressure-area/molecule isotherms of both the DL-12-hydroxyoctadecanoic acid/methyl DL-12-hydroxyoctadecanoate system and the methyl DL-12-hydroxyoctadecanoate/ethyl DL-12-hydroxyoctadecanoate system were measured at a variety of compositions and temperatures. Taking advantage of the thermodynamic threatment which has been formulated by Motomura, the two-dimensional phase diagrams and the apparent molar entrophy, enthalpy and energy changes were evaluated for both systems. The DL-12-hydroxyoctadecanoic acid/ethyl DL-12-hydroxyoctadecanoate system exhibited a deformed cigar type diagram while the methyl DL-12-hydroxyoctadecanoate/ethyl DL-12-hydroxoctadecanoate system formed a normal cigar type diagram. The transition thermodynamic quantities of both the DL-12-hydroxyoctadecanoic acid/ethyl DL-12-hydroxoctadecanoate system and the methyl DL-12-hydroxyoctadecanoate/ethyl DL-12-hydroxyoctadecanoate system were respectively much larger than those of similar monopolar/monopolar systems: hexadecanoic acid/ethyl hexadecanoate and ethyl hexadecanoate/ethyl heptadecanoate.     

Compositional patterns in vertebrate genomes: Conservation and change in evolution

Summary The evolution of vertebrate genomes can be investigated by analyzing their regional compositional patterns, namely the compositional distributions of large DNA fragments (in the 30–100-kb size range), of coding sequences, and of their different codon positions. This approach has shown the existence of two evolutionary modes. In the conservative mode, compositional patterns are maintained over long times (many million years), in spite of the accumulation of enormous numbers of base substitutions. In the transitional, or shifting, mode, compositional patterns change into new ones over much shorter times.The conservation of compositional patterns, which has been investigated in mammalian genomes, appears to be due in part to some measure of compositional conservation in the base substitution process, and in part to negative selection acting at regional (isochore) levels in the genome and eliminating deviations from a narrow range of values, presumably corresponding to optimal functional properties. On the other hand, shifts of compositional patterns, such as those that occurred between cold-blooded and warm-blooded vertebrates, appear to be due essentially to both negative and positive selection again operating at the isochore level, largely under the influence of changes in environmental conditions, and possibly taking advantage of mutational biases in the replication/repair enzymes and/or in the enzyme make-up of nucleotide precursor pools. Other events (like translocations and changes in chromosomal structure) also play a role in the transitional mode of genome evolution.The present findings (1) indicate that isochores, which correspond to the DNA segments of individual or contiguous chromatin domains, represent selection units in the vertebrate genome; and (2) shed new light on the selectionist-neutralist controversy.This work was presented at the EMBO Workshop on Evolution (Cambridge, UK, 4–6 July 1988) and at the 16th International Congress of Genetics (Toronto, Canada, 20–27 August 1988)     

Thermodynamics of the daunomycin-DNA interaction: ionic strength dependence of the enthalpy and entropy

Fluorescence and absorbance methods were used to study the interaction of daunomycin with calf-thymus DNA over a wide range of temperatures and NaCl concentrations. van't Hoff analysis provided estimates for the enthalpy of the binding reaction over the NaCl range of 0.05–1.0 M. Daunomycin binding is exothermic over this entire range, and the favorable binding free energy arises primarily from the large, negative enthalpy. Both the enthalpy change and entropy change are strong functions of ionic strength. Possible molecular contributions to the enthalpy and entropy are discussed, leading to the tentative conclusion that hydrogen-bonding interactions at the interacalation site are the primary contributors to the observed thermodynamic parameters. The dependence of the enthalpy on the ionic strength is well beyond the predictions of current polyelectrolyte theory and cannot be fully accounted for. The enthalpy and entropy changes observed compensate one another to produce relatively small free-energy changes over the range of solution conditions studied.     

Spectral Tuning of Plasmon Resonances of Bimetallic Noble Metal Alloy Nanoparticles Through Compositional Changes

The optical absorption properties of the bimetallic noble metal alloy (viz. Au-Ag, Au-Cu, and Ag-Cu) nanoparticles (NPs) of radii of 10 nm and 20 nm embedded in silica glass have been studied theoretically using a simple model based on the effective medium theory. Our study reveals that the spectra of the above bimetallic alloy NPs exhibit single but composition-sensitive surface plasmon resonance (SPR) peak which indicates the successful formation of alloys. The position of the SPR peak that appeared corresponding to alloy NPs is different from that of the component metals. The study further reveals that the Ag-Au and Au-Cu alloy systems are completely miscible over the entire concentration range but Ag-Cu is miscible up to a certain extent, although, their SPR peak shows a linear shift with molar concentration. It has been further observed that the phase of the Ag-Au alloy system changes with concentration of Au during the alloy formation but no such change is seen in the other two systems. Thus, our study shows that the Ag-Cu system which otherwise does not form alloy in bulk may form alloy in nanoscale with limited miscibility. A shift of the SPR peak positions from ~ 405 to ~ 535 nm for Au-Ag, from ~ 535 to ~ 590 nm for Au-Cu, and from ~ 405 to ~ 436 nm for Ag-Cu NP systems has been observed for different composition of constituent monometals. The compositional changes lead to a spectral tuning of the SPR of the system under studies.

     

Coexistence of two liquid crystalline phases in dihydrosphingomyelin and dioleoylphosphatidylcholine binary mixtures

Recently, DHSM, a minor constituent in naturally occurring SMs, was indicated to form a raft-like ordered phase more effectively than a naturally occurring form of SM because DHSM has greater potential to induce the intermolecular hydrogen bond. In order to examine the influence of the DHSM-induced hydrogen bond on the phase segregation, the thermal phase behavior of stearoyl-DHSM/DOPC binary bilayers was examined using calorimetry and fluorescence observation and compared with that of SSM/DOPC binary bilayers. Results revealed that the DHSM/DOPC bilayers undergo phase segregation between two L_α phases within a limited compositional range. On the other hand, apparent phase separation was not observed above main transition temperature in SSM/DOPC mixtures. Our monolayer measurements showed that the lipid packing of DHSM is less perturbed than that of SSM by the addition of small amount of DOPC, indicating a stronger hydrogen bond between DHSM molecules. Therefore, in DHSM/DOPC binary bilayers, DHSM molecules may locally accumulate to form a DHSM-rich domain due to a DHSM-induced hydrogen bond. On the other hand, excess accumulation of DHSM should be prevented because the difference in the curvature between DHSM and DOPC assemblies causes elastic constraint at the domain boundary between the DHSM-rich and DOPC-rich domains. Competition between the energetic advantages provided by formation of the hydrogen bond and the energetic disadvantage conferred by elastic constraints likely results in L_α/L_α phase separation within a limited compositional range.     

Mutants of thermotaxis in Dictyostelium discoideum

Amoebae of Dictyostelium discoideum, strain HL50 were mutagenized with N-methyl-N′-nitro-N-nitrosoguanidine, cloned, allowed to form pseudoplasmodia and screened for aberrant positive and negative thermotaxis. Three types of mutants were found. Mutant HO428 exhibits only positive thermotaxis over the entire temperature range (no negative thermotaxis). HO596 and HO813 exhibit weakened positive thermotaxis and normal negative thermotaxis. The weakened positive thermotactic response results in a shift toward warmer temperatures in the transition temperature from negative to positive thermotaxis. Mutant HO209 exhibits weakened positive and negative thermotactic responses and has a transition temperature similar to the ‘wild type’ (HL50). The two types of mutants represented by HO428, HO596 and HO813 support the model that positive and negative thermotaxis have separate pathways for temperature sensing. The type of mutants which contains HO209 suggests that those two pathways converge at some point before the response.     

The influence of low amounts of cholesterol on the interdigitated gel phase of hydrated dihexadecylphosphatidylcholine.

Dihexadecylphosphatidylcholine (DHPC)/cholesterol binary mixtures in excess of water have been characterized by small-angle X-ray diffraction and differential scanning calorimetry and a temperature-composition phase diagram for this binary has been constructed. The property of cholesterol to perturb the hydrocarbon chain interdigitation in the lamellar gel phase of DHPC and to convert it into a non-interdigitated state has been observed by small- angle X-ray diffraction at cholesterol concentrations as low as 0.1 mol%. The interdigitated and non-interdigitated lamellar gel phases coexist in the range up to 5 mol% cholesterol. At this and higher cholesterol concentrations only non-interdigitated phases have been found in the phase diagram of the mixture. It is suggested that the ability of cholesterol in low concentration to eliminate the hydrocarbon chain interdigitation is related to the free energy increase due to unfavourable line boundaries between the interdigitated and non-interdigitated lipid domains.